In a radio communication, a diversity receiving method for receiving a signal by using a plurality of antennas is considered effective to reduce an influence of reception electric field reduction due to fading. The radio device that employs the above-described method will be described in the following prior art document.
There is a known selection diversity receiving method for receiving a signal with an antenna with a higher reception electric field from among the plurality of antennas. According to the selection diversity receiving method, since one system of equipment for reception in a device is used to correspond to a plurality of antennas, the cost of the device configuration may be reduced. According to the selection diversity receiving method, a reception electric field of each antenna in the reception time of the preamble of the radio packet is measured, for example. The control is performed so that the antenna with a higher reception electric field is switched as an antenna for reception. Accordingly, to select a proper reception antenna, a sufficient time for measuring the reception electric field strength is desired. In other words, a preamble with a sufficient length is desired.